In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks, CNs. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Device discovery is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples comprise Bluetooth and several variants of the IEEE 802.11 standards suite, such as, e.g. WiFi Direct. These systems operate in the unlicensed spectrum.
Recently, D2D communications as an underlay to cellular or radio communications networks have been proposed as a means to take advantage of the proximity of communicating devices, i.e. UEs, and at the same time to allow devices to operate in a controlled interference environment.
Typically, it is suggested that such D2D communication should share the same spectrum as the cellular or radio communication network. This may be performed, for example, by reserving some of the cellular or radio uplink, UL, resources for D2D communication purposes. Another solution may comprise allocating a dedicated spectrum for D2D communication, which is a less likely alternative as spectrum is a scarce resource; particularly, since dynamic sharing between D2D services and cellular/radio services is more flexible and provides a higher spectrum efficiency.
It becomes clear that for D2D communication to occur, the UE must have the same understanding of UL subframe timing as the cellular or radio communications network. Otherwise, they might overlap in time with the cellular or radio transmissions.
In LTE, like in several other cellular standards, a so-called Timing Advance, TA, is used to ensure that UL transmissions from different UEs are received at the same time, approximately, at the base station. Thereby, orthogonality between the UEs is maintained.
In essence, the base station is measuring the arrival time of transmissions from the UEs and, when necessary, transmitting a timing advance command to the UEs to adjust the transmission timing. At the UE, the timing of downlink, DL, transmissions is known, that is, since the UE is capable of receiving DL transmissions it has established a DL timing reference. The TA command received by the UE is used to determine the start of an UL subframe relative to the start of a DL subframe, i.e. the UL timing reference is obtained from the DL timing reference and the TA command. The propagation delay from the base station to UEs far out in a cell is larger, and therefore a larger TA is needed, compared to UEs that are located close to the base station. This is illustrated in FIG. 1.
To maintain orthogonality between the transmissions from different UE, the timing misalignment at the base station should be, significantly, less than the duration of the cyclic prefix. In OFDM-based systems, such as, e.g. LTE, a cyclic prefix is commonly used to handle time dispersion in the radio channel. Note that the cyclic prefix preferably should cover the time dispersion in the channel as well, and that the timing misalignment allowed must take this into account. If the UE has not received a TA command in a configurable time period, the UE declares the UL not to be time synchronized. This may be implemented by starting a timer, such as, a Timing Advance Timer, TAT, at each reception of a TA command. When the TAT expires, e.g. reaches zero, the UL is considered no to be time aligned.
Cellular systems, or radio communications networks, often define multiple states for the UE which matches different transmission activities. For example, in LTE, two states are defined:
RRC_IDLE, where the UE is not connected to a particular cell and no data transfer may occur in either UL or DL. In this state, the UE is in Discontinuous Reception (DRX) most of the time except for occasionally monitoring the paging channel. RRC stands for Radio Resource Control.                RRC_CONNECTED, where the UE is connected to a known cell and may receive DL transmissions. Although expressed differently in the standard specifications, this state may be considered to have two sub-states:        UL_IN_SYNC, where the UE has a valid TA value such that UL transmissions may be received without collisions between different UEs; and        UL_OUT_OF_SYNC, where the UE does not have a valid TA value and hence cannot transmit data in the UL. Here, prior to any transmission, a random access must be performed to synchronize the uplink.        
Furthermore, in LTE, random access is used to achieve UL time synchronization for a UE which either has not yet acquired, or has lost, its UL synchronization. Once UL synchronization is achieved for a UE, the base station, in this case, a eNodeB, may schedule orthogonal UL transmission resources for the UE.
Some examples of relevant scenarios in which the Random Access Channel, RACH, is used for the random access are therefore:                A UE in RRC_CONNECTED state, but not UL synchronized, needing to send new UL data or control information, such as, for example, an event-triggered measurement report or a hybrid Automatic Repeat Request (ARQ) acknowledgement in response to a DL data transmission;        A UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell;        For positioning purposes in RRC_CONNECTED state, when TA is needed for UE positioning;        A transition from RRC_IDLE state to RRC_CONNECTED, such as, for example, for initial access or tracking area updates;        Recovering from a Radio Link Failure, RLF.        
For D2D communication, it is necessary to define the transmission and reception timing. In principle, any transmission timing could be used as long as transmissions do not interfere with cellular communication. One solution is to use the same transmission timing at the UE for D2D transmissions as for cellular UL transmissions. This ensures that D2D transmissions do no collide with UL transmissions from the same UE, and also avoids a, potentially complicated, additional TA mechanism for the D2D communication.
Please note that the term ‘cellular’ as used herein could be further extended to an out-of-network coverage scenario, where the UEs may establish a hierarchical structure consisting of UE cluster head, CH, i.e. one UE serving as the CH, and slave UEs controlled by the UE serving as the CH. In this way, the CH in many respects behaves similar to a base station, or eNB in this case, and the concept of ‘cluster’ may be seen as the ‘cell’ in traditional cellular or radio communications network. Hence, in the following, the term ‘cellular’ may be also applied to the hierarchical structure of UEs comprising a CH and slave UEs.
Even in absence of a cellular connection, i.e. when the UE is in an RRC_IDLE mode, UEs may perform both D2D peer discovery and/or D2D communication data transmissions on a reserved resource pool in order to save signalling overhead for arbitrary control. Furthermore, despite that a UE does not need to maintain UL timing after the TAT has expired; UEs may still want to send D2D data. However, these communications may then add interference within the cell.